The invention relates to an electronic control system for controlling the operation of an automatic, variable speed transmission in which a speed stage is changed by controlling the energization of solenoid valves disposed in a hydraulic control circuit of the transmission, and more particularly, to a control of the pressure in the hydraulic control circuit during a shift operation.
In an automatic, variable speed transmission having a torque converter, it is necessary that an oil pressure servo to which an oil pressure is supplied and another oil pressure servo from which the pressure is discharged be supplied and exhausted at an appropriate time before and after a shift operation. A conventional practice is to use an accumulator in order to control the timing and the oil pressure. However, the accumulator is incapable of providing a sufficient control, leaving much to be improved in preventing a gear idling or impulses during a shift operation. To accommodate for this problem, an automatic transmission of an electronically controlled type has solenoid valves disposed in its hydraulic circuit which regulate the oil pressure by periodically discharging the pressure from oil paths in response to an output from an electrical control circuit which causes the solenoid valves to be opened or closed during a shift operation, thus preventing the occurrence of impacts. In a control system which utilizes the hydraulic pressure control mechanism of this type, the pressure is controlled by a plurality of solenoid valves which operate to regulate the oil pressure, for example, during a manual N.fwdarw.D shift and automatic 1.fwdarw.2, 2.fwdarw.3 and 3.fwdarw.4 shifts of a four gear, automatic transmission. However, a number of problems are presented with this arrangement in that there must be provided an increased number of electrical circuits which are disposed in one to one correspondence to each of the solenoid valves and which are periodically controlled to open or close, with the length of period varying from circuit to circuit, resulting in a complexed arrangement of the individual circuits for which an individual adjustment is required.